1. Field of the Invention
The invention relates to a method, which allows a phase-stable frequency change of the signal generator and of the local oscillator, for example, of a network analyzer. Within the context of the present application, this method is referred to as the Secum-Trahenz method.
2. Related Technology
A vectorial network analyzer with controllable signal generators and controllable oscillators is known from DE 102 46 700 A1. Only a single local oscillator is often provided in the network analyzers used in practice. Several signal generators are generally provided in more-recent network analyzers with more than two test ports. For example, with a network analyzer comprising three test ports, two signal generators are present, one of which can be switched between two of the three test ports. A mixer, which is connected to a directional coupler, by means of which the wave travelling outwards via the test port or respectively the wave travelling inwards via the test port is de-coupled and fed into the mixer, is provided for each test port. The other input of the mixer is connected to the local oscillator.
With network analyzers and also other measuring instruments, for example, with connected signal generators, it is problematic that a frequency change of the signal generators has to be implemented in a phase-stable manner, that is to say, no sudden phase changes should occur during the frequency change. Signal generators generally comprise synthesisers with several phase-locked-loop (PLL) stages with fractional dividers. A division by a fractional-rational division factor is implemented in the dividers. If the division factor is varied not exclusively with regard to its integer component, this generally leads to a sudden phase change. Even if only the integer division factor is varied, a sudden phase change by π/n, where n is a whole number, can occur. The necessity for implementing a phase-stable frequency change is relevant particularly when measuring frequency-converting devices under test, such as mixers.